1) Field of the Invention
The present invention relates to selective processing of a workpiece and, more particularly, to a method of providing stop-off on select portions of the workpiece so that the workpiece can be selectively diffusion bonded or otherwise processed.
2) Description of Related Art
Stop-off generally refers to a material disposed on a workpiece for facilitating subsequent processing of select portions of the workpiece. For example, according to one conventional diffusion bonding (DB) process, stop-off is provided in a predetermined pattern between workpieces to prevent bonding of corresponding portions of the workpieces. In particular, the stop-off is typically disposed in a select pattern on a surface of a first workpiece on one or more portions of the workpiece where bonding is not desired. Thereafter, a second workpiece is positioned against the surface of the first workpiece so that the stop-off is between the two workpieces. The workpieces are pressed together and heated to a diffusion bonding temperature so that diffusion bonds are formed where no stop-off is provided between the workpieces. In those portions where stop-off is provided, the stop-off prevents bonding from occurring. The unbonded portions of the resulting assembly can define any pattern, such as a pattern of cells that are to be expanded subsequently by a gas pressure during superplastic forming (SPF) of the workpieces.
One conventional method for applying the stop-off to the workpiece includes the use of a silk-screening device. The silk-screening device includes a flat screen or template for disposing the stop-off onto the workpiece. That is, the screen defines a perforated portion that corresponds in shape to the areas of the workpiece where the stop-off is to be applied. The other portions of the screen are impervious to the stop-off. Thus, with a first side of the screen directed toward the workpiece, stop-off is spread onto the opposite side of the screen and pushed therethrough and onto the workpiece in the desired pattern.
The conventional silk-screening method is effective for disposing the stop-off but requires the use of the silk-screening machine as well as careful operation and handling of the workpiece, e.g., to prevent the stop-off from being spread onto the clean portions of the workpiece during or after the silk-screening operation. In addition, the precision and accuracy of the conventional silk-screening method is limited. Further, periodic cleaning of the screen is generally required, e.g., between each successive operation of the device. The cleaning operation slows the overall operating speed of the device and typically requires special disposal facilities, as the stop-off can include hazardous chemicals.
According to another conventional process, a strippable mask coating can be provided on the workpiece to prevent the stop-off from being applied to select portions of the workpiece. For example, U.S. Pat. No. 5,513,791 to Rowe, et al. describes a strippable mask coating that is applied to the surface of a sheet. The mask coating is selectively removed from the surface, i.e., from the areas where bonding is not desired, and the stop-off is then applied and dried on the sheet. The remainder of the flexible mask material is then removed. Any stop-off disposed onto the flexible mask material is removed with the mask material. Thus, the method does not require precise placement of the stop-off material. However, the stripping of the strippable mask material from the sheet requires cleaning of the sheet prior to diffusion bonding in order to remove the oily residue of the mask material, e.g., using an aqueous detergent solution. If not cleaned from the sheet, the residue from the mask material can prevent proper diffusion bonding.